The present invention relates to integrated circuit fabrication, particularly to the use of a directed light source, such as laser pantography, in the fabrication of integrated circuits, and more particularly to a three-dimensional (3-D) laser patterning process which involves the use of an isotropically deposited, photodefinable layer, such as electrodeposited photoresist, to pattern the 3-D surfaces.
Patterning in three dimensions has always been a difficult problem. Typical screening and photolithography permits the patterning of two dimensional surfaces with fine line features (screening can be used for patterns with lines greater than 0.002 inches, while photolithography can resolve line widths on the order of 0.5 .mu.m). Neither process can be readily adapted for vertical surface patterning. Directed light sources, such as lasers, can be used to pattern three-dimensional surfaces. The exposure source (the laser beam) can be manipulated (steered) to any appropriate surface through the use of computer controlled optics and stages. The laser is not used simply as a light source that shines through a mask as in conventional lithography, but is focused to the required size and rastered with computer control to form the pattern which has been stored as a computer file. This three dimensional laser patterning is sometimes referred to as Laser Pantography or laser direct-writing.
The laser must interact with a light or heat sensitive material in order to form a pattern. Laser pantography can be used with a large number of different chemical/physical systems to form the patterns required. The earliest systems involved gases, such as nickel carbonyl, which deposited metal in the areas in which the laser scanned. These processes were extremely slow (.mu.m/sec time frame), and therefore have found utility only for niche applications (e.g. circuit repair) in the semiconductor industry. Other processes use the laser to etch a maskant material to form the pattern. This type of process is considerably faster, but cannot compete with the speed of screening or conventional lithography. Both gas phase deposition and etching require noxious gases in enclosed environments. In recent years chemical transformation of seed layers with laser exposure has provided greater patterning speed and a cleaner process environment.
One such material is photoresist, which is chemically transformed by light to form a desired pattern. The problem with resist is that it is usually spun onto flat surfaces. In order to pattern three-dimensions, the resist must conformally cover vertical and horizontal surfaces equally well.
Laser pantography (computer controlled laser direct-write patterning) is currently used to pattern metal interconnections of various kinds. In particular, laser pantography is being considered for interconnecting integrated circuit (IC) chips to multichip module substrates (or printed wire board substrates), by patterning the top of the chip, the sidewall of the chip and the surface of the substrate. Laser pantography is also being used to form a continuous pattern on the top, sides and bottom of packaging substrates for the purpose of interconnecting several multichip packages into a single package. For example, thin film wiring can be fabricated down beveled edges of the chips using laser pantography, as described and illustrated in U.S. Pat. No. 4,992,847 issued Feb. 12, 1991 to D. B. Tuckerman.
The interconnection of integrated circuit (IC) chips to substrates (e.g. multichips, modules, printed circuit boards, lead frames) is being accomplished in the industry today primarily with wire bonding, tape automated bonding, or with flip-chip solder-bump bonding. Laser pantography can also be used to interconnect chips to substrates as pointed out above. However, in order for laser pantography to compete with other chips/substrate interconnection techniques, it must be manufacturable (simple, inexpensive, environmentally safe). One method for utilizing laser pantography is to pattern the sidewall of the chip itself, and this technique has been developed, as exemplified by the above-referenced U.S. Pat. No. 4,992,847, and produces a higher density of metal circuits than the other techniques, with better electrical and thermal properties.
For example, a type of photoresist, or photodefinable layer, has been introduced commercially which permits a conformal covering of vertical and horizontal surfaces. It has been recognized that by using this new resist, laser patterning can be carried out in air, and three-dimensional (3-D) laser patterning can be processed very fast compared with earlier laser processes, and is simple and fast enough to be a manufacturable process. By the recognition that this newly introduced photoresist, an isotropically deposited photodefinable layer, can be used in techniques such as laser pantography, the process of this invention has been developed for the interconnection of integrated circuit chips to substrates, as well as laser patterning on surfaces. The process of this invention also allows laser pantography, for example, to be used to pattern the top, side, and bottom of substrates, in order for signals to be brought from one side to the other, and allow substrate-to-substrate interconnection in a multichip module stack, and allows chip-to-chip interconnection in a vertical multi-chip stack.